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<H3>angle_style class2 command 
</H3>
<H3>angle_style class2/omp command 
</H3>
<P><B>Syntax:</B>
</P>
<PRE>angle_style class2 
</PRE>
<P><B>Examples:</B>
</P>
<PRE>angle_style class2
angle_coeff * 75.0
angle_coeff 1 bb 10.5872 1.0119 1.5228
angle_coeff * ba 3.6551 24.895 1.0119 1.5228 
</PRE>
<P><B>Description:</B>
</P>
<P>The <I>class2</I> angle style uses the potential
</P>
<CENTER><IMG SRC = "Eqs/angle_class2.jpg">
</CENTER>
<P>where Ea is the angle term, Ebb is a bond-bond term, and Eba is a
bond-angle term.  Theta0 is the equilibrium angle and r1 and r2 are
the equilibrium bond lengths.
</P>
<P>See <A HREF = "#Sun">(Sun)</A> for a description of the COMPASS class2 force field.
</P>
<P>Coefficients for the Ea, Ebb, and Eba formulas must be defined for
each angle type via the <A HREF = "bond_coeff.html">bond_coeff</A> command as in the
example above, or in the data file or restart files read by the
<A HREF = "read_data.html">read_data</A> or <A HREF = "read_restart.html">read_restart</A>
commands.
</P>
<P>These are the 4 coefficients for the Ea formula:
</P>
<UL><LI>theta0 (degrees)
<LI>K2 (energy/radian^2)
<LI>K3 (energy/radian^3)
<LI>K4 (energy/radian^4) 
</UL>
<P>Theta0 is specified in degrees, but LAMMPS converts it to radians
internally; hence the units of the various K are in per-radian.
</P>
<P>For the Ebb formula, each line in a <A HREF = "bond_coeff.html">bond_coeff</A>
command in the input script lists 4 coefficients, the first of which
is "bb" to indicate they are BondBond coefficients.  In a data file,
these coefficients should be listed under a "BondBond Coeffs" heading
and you must leave out the "bb", i.e. only list 3 coefficients after
the angle type.
</P>
<UL><LI>bb
<LI>M (energy/distance^2)
<LI>r1 (distance)
<LI>r2 (distance) 
</UL>
<P>For the Eba formula, each line in a <A HREF = "bond_coeff.html">bond_coeff</A>
command in the input script lists 5 coefficients, the first of which
is "ba" to indicate they are BondAngle coefficients.  In a data file,
these coefficients should be listed under a "BondAngle Coeffs" heading
and you must leave out the "ba", i.e. only list 4 coefficients after
the angle type.
</P>
<UL><LI>ba
<LI>N1 (energy/distance^2)
<LI>N2 (energy/distance^2)
<LI>r1 (distance)
<LI>r2 (distance) 
</UL>
<P>The theta0 value in the Eba formula is not specified, since it is the
same value from the Ea formula.
</P>
<HR>

<P>Styles with a <I>cuda</I>, <I>gpu</I>, <I>omp</I>, or <I>opt</I> suffix are functionally
the same as the corresponding style without the suffix.  They have
been optimized to run faster, depending on your available hardware, as
discussed in <A HREF = "Section_accelerate.html">Section_accelerate</A> of the
manual.  The accelerated styles take the same arguments and should
produce the same results, except for round-off and precision issues.
</P>
<P>These accelerated styles are part of the USER-CUDA, GPU, USER-OMP and OPT
packages, respectively.  They are only enabled if LAMMPS was built with
those packages.  See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A>
section for more info.
</P>
<P>You can specify the accelerated styles explicitly in your input script
by including their suffix, or you can use the <A HREF = "Section_start.html#start_7">-suffix command-line
switch</A> when you invoke LAMMPS, or you can
use the <A HREF = "suffix.html">suffix</A> command in your input script.
</P>
<P>See <A HREF = "Section_accelerate.html">Section_accelerate</A> of the manual for
more instructions on how to use the accelerated styles effectively.
</P>
<HR>

<P><B>Restrictions:</B>
</P>
<P>This angle style can only be used if LAMMPS was built with the CLASS2
package.  See the <A HREF = "Section_start.html#start_3">Making LAMMPS</A> section
for more info on packages.
</P>
<P><B>Related commands:</B>
</P>
<P><A HREF = "angle_coeff.html">angle_coeff</A>
</P>
<P><B>Default:</B> none
</P>
<HR>

<A NAME = "Sun"></A>

<P><B>(Sun)</B> Sun, J Phys Chem B 102, 7338-7364 (1998).
</P>
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